Industrial refrigeration at low temperatures of the order of -110.degree. C. has expanded considerably in recent years. A considerable proportion of this is for laboratory equipment, cryostats and test chambers for the testing of materials at low temperatures. However, refrigeration plants for the temperature range mentioned are being increasingly used in medicine, chemistry and process engineering and also in the refining of steel and, in addition, are required for relatively high outputs.
Simple mineral oils or normal naphthenic oils have long been used as lubricants in refrigeration plants although they have been unsatisfactory in many respects (no extreme temperature stability, highly variable viscosity).
Semisynthetic oils, fully synthetic aromatic alkyl compounds, poly-.alpha.-olefins, provided further improvements. Synthetic liquid lubricants on a chemical basis, which were a move away from mineral oil, for example butyl esters of polysilicic acids, were the outcome of this development. However, liquid lubricants of the type in question show a certain sensitivity to hydrolysis which, on contact with water, can cause the lubricant to gel.
Using organopolysiloxanes as lubricants would be one way of avoiding sensitivity to hydrolysis. However, it is generally known that dimethyl polysiloxanes for example are basically poor lubricants although their viscosity temperature behavior could offer advantages for a liquid lubricant in low-temperature plants.
Although phenyl-, halophenyl- and trifluoropropyl-substituted siloxanes have good lubricating properties, they show poor viscosity temperature behavior compared with dimethyl polysiloxanes (cf. J. of Chem. and Engng. Data 6 (1961), 155). Although siloxanes such as these were used in the production of high-temperature lubricating greases, they were never used as low-temperature lubricants.
U.S. Pat. No. 3,642,626 also describes lubricants which are stable over a very wide temperature range. The lubricants in question are polymeric fluorosilicones having relatively high degrees of polymerization which, on account of their high viscosity at low temperatures, afford no improvement as low-temperature lubricants. In addition, their production is complicated and expensive.
To counteract this problem, DE-OS 27 50 980 proposes a fluorine-containing siloxane corresponding to the following general formula EQU R.sub.F R.sub.2 SiOSiR.sub.2 R.sub.F
in which R.sub.F represents the group C.sub.n F.sub.2n+1 --CH.sub.2 --CH.sub.2 --(n=1-12).
This oil may be used individually or, optionally, in admixture with lubricants known per se.
Practical tests with this oil have shown that it has advantages both in the production of low-temperature lubricants and in the production of greases in accordance with DE-OS 1 769 094. However, this low-temperature lubricant has a low initial viscosity which makes it unsuitable for certain applications. It has in fact been found that lubricating oils which should preferably lie in the ISO VG 46 viscosity class are required, i.e. the lubricating oils must have kinematic viscosities of 41.4 to 50.6 mm.sup.2 /sec at 40.degree. C.